(a) Field of Invention
The present invention relates to papermaking and more particularly to pressing a sheet, such as paper, using at least four press rollers disposed in contact with one another along a circuit as a closed train to define a sealed chamber therebetween for the passage of air thereto and through the sheet via a pervious roller wall.
(b) Description of Prior Art
In papermaking, a wet slurry of pulp is formed into a continuous sheet by drainage at travelling-mesh means, then the formed sheet is pressed between rollers for more drainage, after which the sheet is dried by heat. As is well known in the art, removal of moisture by pressing is less costly than removal by heat. This is mainly because moisture removal by pressing utilizes little space, equipment and energy compared to moisture removal by heat. Heating is costly in the main because fuel is costly; a chain of equipment from fuel tank to paper-machine dryers comprises a great part of a paper mill, while a chain of energy exchanges from fuel tank to vapor exhaust at paper-machine dryers lose energy at every stage of energy exchange.
The present invention is directed toward removing moisture by the less costly means, namely pressing. As is also well known, pressing a sheet helps to interlock fibers for paper strength, and the present invention therefore is directed toward providing as many nips as possible in a small space. Multi-nip rollers are known for pressing a sheet to remove moisture thereof as is disclosed in U.S. Pat. No. 4,173,249 issued on Nov. 6, 1979 to Holkko. The present invention is an improved multi-nip suction press which overcomes some of the disadvantages of the prior art such as taught by the above referred to patent. The disadvantage of using a liquid press medium as in this reference is that it causes the sheet to re-pulp as it passes through the nips, as some of the water being squeezed out is trapped between the impervious sheets and opposed rollers forming the liquid medium chamber and redistributed into the pulp. Also, there is no means to extract the water from the nips and in the case of the upper nips, water will build up in that region as it cannot be released by gravity.